1. Field of the Invention
The present invention relates to a method of preparing a high-temperature superconducting wire, and more particularly, it relates to a method of preparing an oxide superconducting wire, which is applied to a high-temperature superconducting coil or the like, having a high critical current density at the liquid nitrogen temperature.
2. Description of the Background Art
With discovery of yttrium, bismuth and thallium oxide superconducting materials respectively exhibiting critical temperatures (Tc) of 90 K, 108 K and 125 K, which are higher than the liquid nitrogen temperature (77.3 K), application of oxide superconducting materials to the fields of energy and electronics are now being expected. In particular, preparation of a wire from an oxide high-temperature superconductor has been positively developed since the discovery of such a material, for the purpose of application to the field of energy. Among various methods which have been studied in relation to such preparation of superconducting wires, there is a method of covering an oxide superconductor with a stabilizing metal for preparing a wire. According to this method, a silver sheath which is charged with an oxide superconductor is subjected to plastic working such as wire drawing and rolling, and thereafter sintered to provide a wire. In this process, high orientation is supplied to the superconductor which is covered with silver by the combination of plastic working and sintering, thereby implementing a high critical current. When a high-temperature superconducting wire is applied to a coil for a magnet, it is possible to take a process of heat treating a long wire which is obtained by charging a silver sheath with an oxide superconductor and performing plastic working, further performing plastic working, and coiling the same to heat treat the coil again. It has been desired that the superconducting coil obtained through such a process maintains a high critical current density along its overall length, in addition to a high critical temperature.
In the aforementioned process, a tape wire which is obtained after the first plastic working is generally coiled and heat treated. In such heat treatment of the wire, it is necessary to prevent adjacent turns of the coiled wire from adhesion to each other, and from diffusion of components therebetween. Japanese Patent Laying-Open No. 4-274115 (1992) discloses a method of heat treating a superconducting wire of a high transition temperature (high Tc) having a silver protective clad layer, by bonding a layer consisting of ceramics onto the silver clad layer. FIG. 1 shows an apparatus which is disclosed in this gazette. Referring to FIG. 1, a superconducting wire 31 having the protective clad layer is supplied from a spool 35, coated with a paint containing ceramics and a cellulose binder in a container 34, and wound on another spool 60 as a wire 32. The wire 32 coated with the ceramics paint is wound on the spool 60 as a parallel coil (solenoid-shaped coil) 70. A metal mesh 62 is arranged between the wire 32 and a peripheral surface 64 of the spool 60, so that stress which is caused by difference in thermal expansion between the wire 32 and the spool 60 can be absorbed. The wire 32 wound on the spool 60 is transferred to a heating furnace, and heat treated for sintering.
During the heat treatment, the ceramics contained in the paint forms diffusion preventing layers between adjacent turns of the coiled wire 32. The spool 60 provided with the wire 32 is horizontally arranged in the furnace with respect to the gravitational direction. During the heat treatment, the respective turns of the solenoid-shaped coil 70 are loosened due to thermal expansion. Following such loosening, the wire 32 hangs down by its own weight. As the result, the wire 32 may be deformed by its own weight. Such deformation reduces superconductivity of the wire 32, particularly its critical current density. When the wire 32 coated with the paint is heat treated for sintering, further, the components of the paint strongly adhere to the wire 32. Thus, it is extremely difficult to separate the paint components from the wire 32 for exposing the silver clad layer on the wire 32 after the heat treatment.
When the tape-shaped wire which is obtained after the first plastic working is heat treated in a furnace in the aforementioned process of preparing a silver sheath oxide superconducting wire, on the other hand, gas may be partially generated from the charged powder to partially expand the silver sheath. Further, a spatial range which can uniformly set heating temperature distribution in the heating furnace is generally limited to some extent, and hence it is difficult to perform the heat treatment under uniform conditions as the wire is increased in length. Such partial expansion of the wire or ununiform heat treatment leads to ununiformalization of the subsequent plastic working, resulting in portions having low critical current densities in the finally obtained coil. In order to prevent such a disadvantage, therefore, improvement of the heat treatment is awaited.
Another method of heat treating a long wire is adapted to heat a wire which is wound in a multilayer manner. According to this method, it is possible to heat the wire in a relatively compact state. Japanese Patent Laying-Open No. 4-329217 (1992) discloses a method of holding a tape wire, which is prepared by covering raw material powder for an oxide superconductor with a metal sheath, between tapes consisting only of ceramics having a smaller thermal expansion coefficient than the metal sheath, coiling the tape wire held between the ceramics tapes, and heat treating the coiled tape wire. This gazette proposes a technique of suppressing thermal expansion of the metal sheath by holding the tape wire with the tapes consisting only of ceramics. When the tape wire is constrained with the ceramics which is different in thermal expansion coefficient, however, stress may be caused in heating and cooling, to deform the tape wire. On the other hand, Japanese Patent Laying-Open No. 58-22335 (1983) discloses a process of winding a metal wire having a surface consisting of copper or a copper alloy on a reel in a multilayer manner while interposing glass cloth or paper between the layers and annealing the wound metal wire. However, no preferable result can be obtained when this technique is applied to preparation of a high-temperature superconducting wire. If a tape of glass cloth is lap-wound with a silver sheath tape wire and heat treated, stress is caused during the heat treatment step similarly to the case of the tape consisting of only ceramics, to bring partial deformation of the tape such as buckling, for example, although diffusion between the layered portions of the silver sheath can be prevented. Further, a relatively thick glass tape is unadvantageous for improving the winding density for heat treating the wire in a more compact state. When paper is employed in place of glass or ceramics, on the other hand, the paper is immediately burnt away under the sintering temperature. Thus, it is impossible to sufficiently suppress diffusion and adhesion between the lapped portions of the silver sheath with paper.